Interesting. I am experiencing exactly the same thing but with a D600. I thought this was related to the D600 not being supported with the current firmware.

I contacted support and provided them with a flow. In my case it seems (not 100% reproducible), that it stops triggering after measuring with the Sekonic. This is the email sent to support:

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>Hi,I contacted support last Friday and it was confirmed that the MiniTT1 and FlexTT5 are not fully compatible with Nikon D600 and that a new firmware is on its way.>I'd like to mention that I have tried the PW on trigger only mode with the Sekonic L-478DR and found the following:>Setup:- 3 SB-800 on 3 FlexTT5- MiniTT1 on D600>All equipment turned on this order: flash - TT5 - Mini - D600>Trigger flashes via L-478DR to measure exposure.Adjust power on flashes via Sekonic if necessary.Take picture from D600. Flashes are not triggeredTurn off MiniTT1 and D600 and then turn them on again.Take picture. Now flashes are triggered all the time.>This behavior is 100% reproducible. Each time I measure exposure with the Sekonic, the Mini/TT5's do not trigger the flashes, forcing me to turn off and then on the Mini and camera.>I am reporting this bug in case your team is not aware of this issue and maybe it can be addressed on the new firmware.>Best regards,

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At the moment of contacting support I thought it was 100% reproducible. Now I am not able to reproduce it on purpose, but stills failing.